The present invention pertains generally to display systems and illumination sources, such as emissive, reflective, or transmissive displays, which may use time-sequential color illumination. More particularly, the present invention relates to drive circuitry for display systems or illumination sources.
In display devices used in computer systems, television sets, instrument panels, and various other devices, the fundamental picture element is often referred to as a pixel. A display screen, or raster, generally comprises a large number of pixels arranged adjacently in a rectangular matrix. In a color display device, each pixel generally has multiple components, or subpixels, each of which displays light of a particular fundamental color. In a red-green-blue (RGB) display system, for example, each pixel includes a red subpixel, a green subpixel, and a blue subpixel. For a given pixel, the combination of the intensities of the subpixels determines the overall color of the pixel perceived by the user. Depending on the type of display device, such as a color cathode ray tube (CRT) or liquid crystal display (LCD), both commonly used for computer monitors and television, the red, green, and blue subpixels are arranged spatially to form a pixel. The subpixels may be produced using any of various technologies, such as color light-emitting diodes (LEDs), LCD elements, phosphors, incandescent lamps, mirrors, etc.
One approach to generating a color pixel is to simultaneously illuminate each of the color subpixels of the pixel at independently-selected intensities. In contrast, in a time-sequential color based display or its illumination system, each pixel is illuminated sequentially in time, rather than simultaneously, during each frame. A frame is the time required to display an entire raster, or more specifically, the time between vertical synchronization pulses in a conventional display device. The subpixels are illuminated so rapidly and close together in time that a person perceives the color of the pixel as a blend of the light output.
Certain problems are associated with prior art display devices and illumination systems. For example, in some LED display devices, a ballast resistor current source is used to power the LEDs. However, display devices which use ballast resistor current sources tend to use power inefficiently and require higher voltages. In addition, in ballast resistor current source drive circuits, the current through an LED varies nonlinearly with both power supply voltage variations and LED forward voltage variations. Consequently, the color balance will vary substantially with even small changes in power supply voltage. Further, it is difficult to adjust the brightness of a color element (i.e., a pixel or illumination source) without affecting: 1) the color balance of the display, pixel, or illumination source, and 2) the uniformity of the display itself.
Refer now to FIG. 1, which illustrates a well-known LED drive circuit of the prior art. Note that the component and parameter values shown are only for the purpose of illustration. As shown, the drive circuit includes a ballast resistor (R1, R2, and R3) in series with each LED of a pixel between the LED (D1, D2, and D3) and the supply voltage. In this circuit, the current through each LED will vary with both power supply voltage and LED forward voltage variations. Because different colored LEDs have different device characteristics and different forward voltage requirements, the color balance will vary significantly with a small change in the power supply voltage. Although color balance might be achieved by individually selecting ballast resistors for each LED during the manufacturing process, to do so would be impractical and costly. Alternatively, color balance might be achieved by using adjustable resistors to adjust the current through each LED. However, again, to do so would be impractical and costly. In addition, blue LEDs, in particular, tend to require a higher forward voltage to operate than red or green LEDs. With the circuit of FIG. 1, the blue LED will not operate if the supply voltage falls below approximately 4.5 volts. Moreover, the ballast resistors R1, R2 and R3 tend to dissipate excessive amounts of power.
FIG. 2 illustrates another well-known LED drive circuit of the prior art. A reference voltage of approximately 1.2 volts, for example, is created using a resistor R4 and two diodes D7 and D8. The reference voltage is applied to the bases of three transistors Q1, Q2 and Q3 to create a constant current source by the voltage across the emitter resistor creating a current and the transistors"" collector and emitter currents being equal. As with the circuit of FIG. 1, color balance might be achieved by individually selecting emitter resistors (R5, R6, and R7) for each current source during the manufacturing process; however, as noted above, that is not a desirable solution. Similarly, while color balance might be achieved by using adjustable resistors to adjust the current through each LED, such a circuit would be difficult to use, impractical, and costly. The color balance and the overall brightness might alternatively be adjusted by using digital-to-analog converters to drive the base of each transistor in the current source. However, that approach would increase the cost of the associated control circuitry. Moreover, as with the circuit of FIG. 1, it is difficult or impossible to turn on the blue LED when the power supply falls below a certain level, i.e., approximately 5.3 volts in the circuit of FIG. 2. Also, as with the circuit of FIG. 1, a significant amount of power is wasted by dissipation in the resistors as well as in the transistors.
Accordingly, what is needed is a color display or illumination source drive circuit that overcomes the above-noted disadvantages of the prior art. Specifically, what is needed is a drive circuit that efficiently drives a display color element (i.e., pixel or illumination source), such as an LED, using a system power supply that can tolerate a variation of supply voltage. Furthermore, it is preferred that color balance be maintained independent of variations in supply voltage or individual pixel or illumination source characteristics. What is further needed is a drive circuit that enables the overall brightness of a display, pixel, or illumination source and the brightness of individual pixels or color illumination sources to be adjusted independently, using easy-to-implement, power-efficient, and inexpensive components and techniques.
One aspect of the present invention is a method and apparatus for controlling the brightness of a pixel or illumination source of a display device independent of color balance.
Another aspect of the present invention is a method and apparatus for operating a pixel or illumination such that its performance is relatively insensitive to variations in power supply or device characteristics.
Another aspect of the present invention is a method and apparatus for operating a color display device that has at least one pixel capable of displaying multiple colors sequentially during each of multiple frames. A drive signal is generated to activate the pixel during each of the frames. The on-time of the drive signal is varied within each of the frames according to which of the colors is being displayed.
In various embodiments, such apparatus may include a boost switching converter or a flyback switching converter. Also, in various embodiments, these and other aspects of the present invention may be applied to emissive, transmissive, or reflective display systems.
Other features of the present invention will be apparent from the accompanying drawings and from the detailed description which follows.